The present invention relates generally to a steering system of a vehicle and more particularly to a method of detecting an absolute steering angle of a steering angle sensor of the vehicle.
The steering angle is decided in accordance with the angle through which a steering wheel (i.e., the steering shaft) is turned in steering action. In a vehicle of this character, it is necessary to provide a steering shaft angle (hereinafter referred to as steering angle) sensor for detecting a rotational angle of the steering shaft. An optical rotary encoder (for example, as disclosed in Japanese Utility Model Laid-Open Publn. No. 51214/1987) has been used as the steering angle sensor.
In the optical rotary encoder, a slitted disk is fixed coaxially to the steering shaft. The slitted disk is provided with a plurality of slits extending radially outward and angularly spaced at a pitch which divides the disk circumference into equal intervals. A photointerrupter with a light emitting element and a photosensitive detector or a photodetector are mounted on a fixed structure such as a steering column on opposite sides of the slitted disk so that a slitted part of the disk is interposed therebetween. Thus, light can be transmitted through the slits from the light emitting element to be received and detected by the photodetector. The light thus transmitted through the slits are detected as pulses by the photodetector when the steering shaft and the slitted disk rotates. As a result, the rotational angle of the steering shaft is detected from the number of the pulses.
The rotary encoder described above generally detects a relative displacement of a steering angle. It cannot however detect an absolute steering angle from an angle of a straight-ahead steering state as a reference some degrees to a certain right or left angle.
Detection of the absolute steering angle is possible by initially setting a straight-ahead steering position of the steering shaft counting the number of pulses of the above mentioned rotary encoder from the straight-head steering position. However, once an electric power supply is cut off, as for example a storage battery is disconnected, a stored memory of the absolute steering angle at that instant is erased. Then, the detection of the absolute steering angle thereafter becomes very difficult.
It is a principal object of the present invention to overcome impossibilities of the detection of the absolute steering angle after the stored memory is erased.